Change "GDExtension example in C++" links to accommodate the new docs file structure.

[Ivorforce]

Depends on godotengine/godot-docs#10631.
Staying as draft until then.

The C++ documentation is moving around a bit, to detangle it from general GDExtension info. There are two hard links referring to the old file structure; as soon as the above PR is merged, the main repo should follow along.

[tetrapod00]

This needs to be merged relatively quickly after godotengine/godot-docs#10631, so that the class reference links are not out of sync. However, the docs PR includes manual changes to the links in the relevant class reference RST files, so as long as this is merged before the next class reference sync action, everything will be in sync.

We'll be merging the docs PR in a week or so, since it should happen only in the 4.5 version of the docs, and we're going to update the 4.4 version of the docs from master one more time first.

See godotengine/godot-docs#10631 (review) for details, and here and here for RC discussion.

[mhilbrunner]

This now needs to be merged ASAP, as godotengine/godot-docs#10631 was merged.

[akien-mga]

Thanks!